Centrifugal compressors that are used in superchargers for reciprocating engines and gas turbine engines are typically provided with an impeller comprising a substantially frusto-conical hub and a plurality of blades having base ends fixedly attached to the hub and defining surfaces that are twisted relative to the central axial line. Impeller design has a strong bearing on the compression efficiency, and various proposals have been made in connection with impeller design. Such an example can be found in Japanese patent laid open publication No. 07-91205.
Each blade defines a suction surface and a pressure surface as it rotates fast with the hub. As can be readily appreciated, the mechanical stress in the blade tends to be high at the base end or hub end thereof. In particular, if the blade surfaces are tilted or leaned with respect to the normal plane as illustrated in FIG. 14, for instance, by a significant angle, the base end or hub end of the blade is subjected to a significant level of mechanical stress. Therefore, it has been customary to avoid the tilting or leaning of the blade, and design the profile of the blade so as to be substantially symmetric with respect to a central normal line (neutral plane) and to have a thickness that decreases linearly from the base end to the tip end thereof as illustrated in FIG. 15, for instance.
As higher output pressure levels (pressure ratios) are demanded from centrifugal compressors, the circumferential speed (rotational speed) and aerodynamic loading of the blades are becoming higher and higher. In particular, when the aerodynamic loading of a blade becomes excessive, particularly in the hub end of the blade, surging may occur owing to aerodynamic separation from the blade, and the efficiency of the compressor may be decreased owing to the generation of secondary flows. Also, the mechanical loading of the blade tends to be increased, and the excessive mechanical stress in the hub end of each blade reduces the durability and reliability of the compressor.
The aerodynamic loading of the hub end of each blade can be mitigated by reducing the aerodynamic loading of the tip end and/or tilting the blade with respect to the normal plane. However, increasing the tilt angle of the blade results in an increase in the mechanical stress of the hub end of the blade. In other words, there is relatively little freedom in controlling the distribution of aerodynamic loading in the radial direction or from the tip end to the hub end of each blade, and this has prevented a further improvement in the performance of compressors for a given size thereof.
In some of the existing centrifugal compressors, one or a plurality of splitter blades each having a relatively receding leading edge are provided between each pair of adjacent full blades. When there is only one splitter blade between each pair of adjacent full blades, the splitter blades are each located centrally between the opposing positive and negative surfaces of the adjoining full blades, and the blade thickness increases linearly from its leading edge in a symmetric manner with respect to the central or neutral plane thereof. Because aerodynamic separation from the leading edges of the adjacent full blades tends occur more actively from the suction surface than the pressure surface, the leading edge of the splitter blade tends to interfere with the separation flow from the suction surface of the adjacent full blade, and this has a damaging effect to the efficiency of the compressor.